The dual purpose of a microphone shield is to block reverberant energy from the room boundaries behind and to the sides of the microphone, while absorbing any sound that enters the device from the rear.
The current state of the art for portable, rigid, stand mounted microphone shields uniformly consists of curved, concave (when viewed from the inside of the device) and/or flat faceted designs that encircle the microphone on three sides. They are all lined with acoustically absorbent material, typically open cell polyurethane foam, or in some cases a layer of nonwoven fibers. The purpose of this absorptive lining is to reduce the level of any internal reflections that may occur from the shell of the shielding device itself caused by direct sound entering the open portion of the shell. This energy can be reflected directly back at the microphone causing unwanted sonic colorations.
There are two design approaches to the structural shell that surrounds the microphone and supports the absorptive material, perforated and solid. The shells that are perforated do a poor job of isolating the microphone from ambient room reflections but to a large degree do not have any, or as much, internal reflections. Solid shells do a much better job of isolating the microphone from the acoustical influence of the room but their mostly semi-circular or concave shape (when viewed from the inside) tend to reflect some level of acoustical energy back to the microphone.